stmmac: fix_mac_speed is called during 10/100<->1000 speed changes

This patch modifies the stmmac_adjust_link() function so the
fix_mac_speed() is called not only when link speed is changing
between 10 and 100 Mbps (as required in RMII mode) but also
for 1000 Mbps.

Signed-off-by: Pawel Moll <pawel.moll@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>